1. Field of the Invention
The present invention relates to a fibre-type light conversion device.
2. Description of Background Information
An optical pickup is known by which high density writing and reading of information on and from a disc are enabled by changing the wavelength of a laser beam emitted from a laser source to a half the original wavelength by means of a light conversion device (reference is directed to Japanese Patent Application Laid-Open No. 61-50122).
As the light conversion device for use in this type of pickup, there is an optical fibre-type SHG (Second Harmonics Generator) using a second-order non-linear optical effect. A phase matching of Cerenkov radiation system is adopted in the optical fibre-type SHG. With this system, it is possible to generate a second harmonic wave (referred to as SH wave hereinafter) whose phase matching condition is attained almost automatically. The general concept of this device is shown in FIGS. 1A and 1B.
In FIG. 1A, when the fundamental wave mode is propagated through the core with the effective reflective index of N (.omega.), the non-linear polarizing wave generating the SH wave is also propagated at the same phase velocity C/N (.omega.) (C is the speed of light). It is assumed that this non-linear polarizing wave produces the SH wave in a direction making an angle .theta. with respect to the direction of the wave guide at a point A, and generates the SH wave in the direction of .theta. as before at a point B, after the elapse of a unit time. If the SH wave generated at the point A propagates through the clad and reaches to a point C after the elapse of the unit time and the angle .theta. is such an angle that lines AC and BC are perpendicular to each other, then the plane of the SH wave which is generated from the non-linear polarized wave between A and B becomes equal to BC, and as a result, a coherent SH wave is generated.
The condition of the phase matching is, according to the figure, as follows: EQU N(.omega.)=N.sub.clad (2.omega.) cos .theta. (1)
where N.sub.clad (2.omega.) is the refractive index of the clad for the SH wave. This in turn gives: EQU N(.omega.)&lt;N.sub.clad (2.omega.) (2)
This means that the SH is generated automatically in the direction .theta. where the phase matching is performed when at least the condition mentioned by the equation (2) is satisfied. Generally, with the refractive indices of the clad and core for the fundamental wave being n.sub.clad (.omega.) and n(.omega.) and with air as the over-layer, the condition for the fundamental wave to propagate through the core as a mode is expressed as follows: EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n(.omega.) (3)
Wavelength dispersion of the clad's refractive index will now be considered. Since n.sub.clad (.omega.) &lt;n.sub.clad (2.omega.), the equation (2) is satisfied for all of the fundamental wave modes irrespectively of the diameter of the core so far as the following expression (4) is satisfied. EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n.sub.clad (2.omega.) (4)
Moreover, there are fundamental modes satisfying the equation (2) in a certain range of the diameter of the core even under the following condition. EQU N.sub.clad (.omega.)&lt;n.sub.clad (2.omega.)&lt;n(.omega.)
The second harmonic wave generated in this way is propagated in a clad mode as illustrated in FIG. 1B in which total reflection occurs repeatedly at the boundary between the clad and air. Then, the second harmonic wave is emitted in conical shape from the end of the fibre in directions determined by the angle .theta. relative to the fibre's direction. The equiphase front of the second harmonic wave emitted in this way is in a conical surface with an axis on the central axis of the fibre.
In order to utilize the second harmonic wave as a light beam for writing and reading information on and from a recording disc as mentioned before, it is necessary to converge the light emerged from the light conversion device on the recording surface of the disk in the form of a small light spot. However, since the equiphase front of the emerging light is in the conical form, it is not possible to converge the emergent light to a degree near to a diffraction limit using a converging lens system made by conventional spherical lenses or aspherical lenses.
Therefore, as illustrated in FIG. 2, it is conceivable to dispose a conical prism 20 with a conical surface in the optical path of the beam emitted from the fibre-type SHG 10, and the conical equiphase front can be converted to a planer equiphase front by collimating the second harmonic (SH) wave (making it parallel) by the function of the conical prism 20. With this arrangement, it becomes possible to converge the light to the diffraction limit by means of a conventional converging lens.
However, because of the conical shape, the production of the conical prism 20 is difficult, and are not suited for the mass-production. Therefore, it is disadvantageous to use a conical lens for reducing the production cost.